The present invention relates to a copolymerized polyester and, more particularly, to a copolymerized polyester which is excellent in transparency and gas barrier property, and is useful as a packaging material in the form of a container and a film.
Polyethylene terephthalate which has excellent physical properties such as mechanical strength, chemical stability, transparency and hygienic quality, and which is light in weight and low in cost, is widely used for a container and a film as a packaging material.
However, the gas barrier property of polyethylene terephthalate cannot be said to be sufficient in some fields, although it is superior to those of other resins such as polyolefin. For example, a packaging material for carbonated beverage, beer and wine is required to have high oxygen gas barrier property and carbon dioxide gas barrier property in order to preserve the content, and a generally used hollow container of biaxially oriented polyethylene terephthalate cannot satisfy those strict conditions on gas barrier properties.
To solve this problem, various methods for improving the gas barrier property of a polyethylene terephthalate container have been proposed. For example, (1) a method of labeling a gas barrier material on a polyethylene terephthalate container or coating the polyethylene terephthalate container with a gas barrier material (Japanese Patent Application Laid-Open (KOKAI) No. 54-117565 (1979)), (2) a method of producing a multi-layered container comprising polyethylene terephthalate and a gas barrier material (Japanese Patent Application Laid-Open (KOKAI) No. 56-64839 (1981)), and (3) a method of producing a container from polyethylene terephthalate with a gas barrier material blended therewith (Japanese Patent Application Laid-Open (KOKAI) Nos. 57-10640 (1982), 59-64658 (1984) and 61-43655 (1986)) have been proposed.
In the methods (1) and (2), polyvinyliden chloride, a saponified ethylene-vinyl acetate copolymer, metaxylene diamine nylon, etc. are exemplified as a gas barrier material. These methods, however, are disadvantageous in the manufacturing process because the step for coating a container with a gas barrier material is added and a new apparatus for producing a multi-layered container is required. The methods (1) and (2) are also disadvantageous in that if the adhesion between the barrier layer and the polyethylene terephthalate layer is poor, a ply separation there between is produced.
On the other hand, the method (3) is advantageous in that it is possible to produce a container having a gas barrier property by utilizing conventional manufacturing apparatus and manufacturing process. However, since a material having a good compatibility with polyethylene terephthalate and a refractive index approximate to that of polyethylene terephthalate is necessary in order to maintain the transparency of a blend, a material is not usable as a gas barrier material in the method (3) merely because it has an excellent gas barrier property. Although polyethylene isophthalate and a copolyester thereof have been proposed as a gas barrier material which satisfies the above-described requirements, the gas barrier properties thereof are insufficient for improving the gas barrier property of polyethylene terephthalate.
As a polyester used as a packaging material in place of polyethylene terephthalate, polyalkylene isophthalate copolymerized with aliphatic dicarboxylic acid having 4 to 12 carbon atoms has been proposed (U.S. Pat. No. 4,403,090), but this polyester is not sufficient for improving the gas barrier property of a polyethylene terephthalate packaging material.
Although a polyester produced by copolymerizing phenylene dioxydiacetic acid and polyethylene terephthalate has been proposed (Japanese Patent Application Laid-Open (KOKAI) No. 60-501060 (1985)), the improvement of gas barrier property cannot be said to be sufficient.
As a gas barrier material having an excellent ply adhesion, polyesters containing isophthalic acid as the main acid component have been proposed (Japanese Patent Application Laid-Open (KOKAI) Nos. 59-39547 (1984), 59-67049 (1984) and 59-89149 (1984)). In these gas barrier materials disclosed in the above-described specification, however, the barrier level is low and in order to produce a container having a sufficient gas barrier property, it is necessary to make the barrier layer thick. The total thickness of the container is, therefore, inconveniently increased. Alternatively, if the polyethylene terephthalate layer is made thin due to the limitation in the total thickness, the mechanical strength and the heat resistance are impaired.
U.S. Pat. No. 4,663,426 also discloses a polyester resin of a diol and a diacid component comprising about 5 to 50 mole % of phenylenedioxydiacetic acid and 50 to about 95 mole % of naphthalenedicarboxylic acid, and further discloses that other well-known polyester-forming diacid such as isophthalic acid, 1,4-cyclohexanedicarboxylic acid, linear lower alkyldicarboxylic acid may be employed. However, this polyester containing a large amount of naphthalenedicarboxylic acid has a high glass transition point (Tg) and is fundamentally a rigid polymer. Further, when this polyester is used as a blend or a laminate with PET, the improvement of the gas barrier property of PET is insufficient.
As a result of the study undertaken by the present inventors to solve the above-described problems in the prior art, it has been found that a copolymerized polyester obtained by copolymerizing a dicarboxylic acid component (A) containing (a) isophthalic acid or an ester forming derivative thereof and (b) a specific phehylenedi(oxyacetic acid) or an ester forming derivative thereof in a specified ratio and a diol component (B) is excellent not only in physical properties such as mechanical strength and transparency but also in gas barrier property. On the basis of this finding, the present invention has been achieved.